Summary of Work: Germline mutations of the BRCA1 and BRCA2 genes confer profound predispositions to breast and ovarian cancer in women. Using gene targeting, we have generated hemizygous Brca2(+/-) mice with an exon 10 truncation. Intercrosses of these Brca2(+/-) mice on several genetic backgrounds (129/SvEv, C57BL/6, DBA2) revealed embryonic lethalty for the Brca2(-/-) genotype. Several experiments are in progress to assess the susceptibility of Brca2-deficient mice to carcinogenic agents. Potential risks associated with mammography are being addressed by exposure to low dose radiation (0.3 Gy) in Brca2(+/-), Brca1(+/-), Brca2(+/-)xBrca1(+/-)F1, Brca2(+/-)xP53(135Val)F1 and six inbred strains of mice. Mammary gland whole mounts from serial sacrifices have been used to monitor preneoplastic alterations. Preliminary results (15 months) indicate that BALB/c and SWR/J mice are most sensitive for mammary tumor formation (17 and 39%, respectively). In a second study, Brca2(+/-), Brca1(+/-) and C3H/He mice were fed diet containing diethylstilbestriol (640 ppb) as a model environmental estrogen. Complete necropsies at 6 months failed to reveal gross mammary tumors and tissues from these mice are currently under pathology review. In a third study, Brca2(+/-) were crossed with Min mice (Apc mutant) and treated with ethylnitrosourea. Initial results indicate that the Brca2 mutation does not act synergistically with Min. Although Brca2(+/-)xMinF1 and Brca2(+/+)xMinF1 develop 6-7 mammary tumors/mouse by 65 days after treatment, only one mammary tumor was observed in 20 Brca2(+/-) mice. To overcome the embryonic lethality of Brca2 (-/-) mice, the Cre/LoxP system for conditional gene targeting is being used to disrupt BRCA2 exon 27 (shown to interact with Rad51). Fifteen chimeric mice with the Brca2 exon 27 flox allele are being bred currently and at least one is capable of germline transmission. Once Brca2flox mice are available, Cre recombinase will be introduced by a variety of approaches to delete exon 27 in mammary gland and possibly other tissues. In addition, we are currently attempting to delete the flox allele in embryonic stem cells by transient transfection with Cre.